The present invention relates to an image forming method using a color printer or color copying machine, and particularly to an image forming method for forming images of colors by means of a plurality of image forming sections arranged in a direction of conveyance of a recording paper sheet.
For example, a color copying machine having an endlessly run conveyor belt and a plurality of image forming sections arranged at regular pitch along the conveyor belt is known as an image forming apparatus. In the color copying machine, a recording paper sheet is held on and conveyed by the conveyor belt, and images of colors are superposed on one another on the recording paper sheet by means of the image forming sections, thereby outputting a color image on the recording paper sheet.
It is considered to be very difficult, in this type of color copying machine, to make the travel speed of the conveyor belt, i.e., the speed of conveying the paper sheet, accurately coincide with the process speed in each of the image forming sections, and to shift the image forming timings in the respective image forming sections by a predetermined time in order to accurately superpose the images of the colors on one another on the recording paper sheet.
For this reason, in the conventional color copying machine, in a warm-up time in which a recording paper sheet is not conveyed (e.g., at the apparatus power-on time or in a reset operation after dealing with a jam), test patterns of the colors are formed on the conveyor belt by means of the respective image forming sections. The test patterns are detected to calculate an amount of deviation of the image forming positions in the respective image forming sections. Based on the calculated amount of positional deviation, the image forming timings or the like in the respective image forming sections are regulated, thereby correcting color drift of the image output to the recording paper sheet.
A wedge-shaped mark is known as a conventional test pattern. The mark has a first line segment extending in the main scanning direction, which is perpendicular to the travel direction of the conveyor belt, and a second line segment extending at an angle from one end of the first line segment. Wedge-shaped marks corresponding to the respective colors are directly formed on the conveyor belt at predetermined positions arranged at regular intervals along the direction of travel of the conveyor belt, i.e., the sub-scanning direction. The wedge-shaped marks corresponding to the respective colors formed on the conveyor belt are detected in sequence by a sensor fixed so as to face the conveyor belt on a side downstream from the image forming sections. Based on the detection result, positional deviation of the image forming positions in the image forming sections is detected with respect to the main scanning and sub-scanning directions. The image forming positions of the respective image forming sections are corrected to correct the positional deviation, so that the color drift of the image can be corrected.
However, according to the conventional color drift correcting sequence using the wedge-shaped mark as described above, it is necessary to sequentially form a plurality of wedge-shaped marks on the conveyor belt in the apparatus warm-up time. In particular, to increase the accuracy of detecting color drift, it is necessary to form a number of wedge-shaped marks preferably over the length of the circumference of the conveyor belt. In this case, a great deal of time is required for the color drift correcting sequence. If a long period of time is required for the color drift correcting sequence, it takes a considerable time to carry out first copy after the power is turned on. Thus, the productivity of the apparatus is lowered.
Further, if the color drift correcting sequence is carried out only at the apparatus warm-up time as described above, it is impossible to correct color drift due to a change with time of a structural element of the apparatus, such as thermal expansion. For example, it is known that one of the rollers around which the conveyor belt is wound is influenced by a fixing apparatus located near the roller, with the result that the diameter of a roller is thermally expanded with the elapse of time. In this case, the travel speed of the conveyor belt is increased, resulting in color drift. Such color drift cannot be corrected by the color drift correcting sequence only at the warm-up time described above.
To correct the color drift resulting from such a change with time, for example, a method is considered in which the interval between recording paper sheets supplied in an image forming operation is extended, the aforementioned wedge-shaped marks are formed between the paper sheets, and color drift in every interval between paper sheets is detected to correct the color drift.
In this method, however, wedge-shaped marks of the number at least the same as the number of the image forming sections must be formed every interval between paper sheets along the sub-scanning direction. Therefore, the interval between the supplied paper sheets must be considerably long. Accordingly, the number of supplied paper sheets on which an image can be formed per unit time is reduced, with the result that the productivity of the apparatus as a whole is inevitably lowered.
Therefore, instead of forming wedge-shaped marks between paper sheets, a method is considered in which wedge-shaped marks are formed along a side end of the conveyor belt outside the region for holding paper sheets. In this method, since the interval between supplied recording paper sheets can be reduced, the number of paper sheets which can be supplied per unit time can be increased. On the other hand, it is necessary that the width of the conveyor belt be wider than that of a recording paper sheet of the maximum size. As a result, the size of the apparatus is inevitably increased.